Monosubstituted benzene, nitration products

In electrophilic aromatic substitution of a monosubstituted benzene, three products are possible the new group may become oriented ortho, meta, or para to the existing group. Table 22.1 shows the orientation of nitration of a series of monosubstituted benzenes. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Holleman [55] gives the following data on the composition of the nitration products obtained in the nitration of different monosubstituted benzene derivatives with mixtures of nitric and sulphuric acids (Table 2). As appears from the data shown below, the substituent already present affects the orientation of the group which is being introduced. It is evident that nitration can be influenced by the steric factor. For exampl tert.-butylbenzene is mainly nitrated in para (72.7%) and to a much lesser extent in ortho (15.8%) positions (H. C. Brown and Nelson [88]). 

Nitration of aromatic compounds such as toluene is important for the synthesis of small-molecule precursors for further use in fine chemicals such as dyes, pharmaceuticals, perfumes, plastics, and explosives. Typically, it is the para nitration product of monosubstituted benzene compounds that is the most desirable. The traditional aromatic nitration method that is still widely used today is the homogeneous... 

Using Menke s conditions, Smith et al.[29,30] have described a method for the nitration of benzene, alkylbenzenes and halogenobenzenes using zeolites with different topologies (HBeta, HY, HZSM-5 and HMordenite) as catalysts and a stoichiometric amount of nitric acid and acetic anhydride. The reactions were carried out without solvent at temperatures between -50 °C and 20 °C. For the nitration of toluene, tridirectional zeolites HBeta and HY were the most active catalysts achieving >99 % conversion in 5 min reaction time. However, HY exhibited selectivity to the p-nitrotoluene very similar to the homogeneous phase, while with HBeta, selectivities to p-nitrotoluene higher than 70% could be achieved. HBeta zeolite exhibited excellent para-selectivity for the nitration of the different monosubstituted aromatics (Table 5.1). The catalyst can be recycled and the only by-product, acetic acid, can be separated by vacuum distillation. 

Chlorobenzene and bromobenzene, for example, undergo nitration at a rate approximately 30 times slower than benzene. The relative percentages of monosubstituted products that are obtained when chlorobenzene is chlorinated, brominated, nitrated, or sulfonated are shown in Table 15.1. 

The electrophilic aromatic substitutions that we studied in Sections 15-9 and 15-10 can be stopped at the monosubstitution stage. Why do Friedel-Crafts alkylations have the problem of multiple electrophilic substimtion It is because the substituents differ in electronic structure (a subject discussed in more detail in Chapter 16). Bromination, nitration, and sulfonation introduce an electron-withdrawing group into the benzene ring, which renders the product less susceptible than the starting material to electrophilic attack. In contrast, an alkylated benzene is more electron rich than unsubstituted benzene and thus more susceptible to electrophilic attack. 

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